The manufacture of semiconductor devices such as LSIs uses lithography technology to form patterns on semiconductor wafers. More specifically, exposure light is directed through a patterned exposure substrate onto a photoresist film deposited on the semiconductor wafer, to transfer the pattern onto the semiconductor wafer. When there are dust particles attached to the exposure substrate, it may result in, for instance, the deformation of the transferred pattern, giving rise to reduced semiconductor device performance or reduced manufacturing yield. Therefore, a pellicle—a dustproof cover that exhibits very high light transmittance—is attached to the surface of the exposure substrate in order to avoid possible attachment of dust particles to the exposure substrate. Even when dust particles are attached to the pellicle, deformation of the transferred pattern does not occur because exposure light has been focused on the exposure substrate.
By way of example, as illustrated in FIG. 3, pellicle 10 includes pellicle membrane 12 that is transparent to exposure light, and pellicle frame 14 having pellicle membrane 12 attached. The material of pellicle membrane 12 is typically nitrocellulose, cellulose acetate or fluorine resin, for example. The material of pellicle frame 14 is typically aluminum, stainless steel or polyethylene, for example. Pellicle membrane 12 and pellicle frame 14 may be bonded together via adhesive layer 13.
Pellicle 10 further includes sticky layer 15 provided on the bottom part of pellicle frame 14. Pellicle 10 is attached to an exposure substrate via sticky layer 15. Sticky layer 15 is made of a binder such as polybutene resin, polyvinyl acetate resin, acrylic resin or silicone resin. Pellicle 10 may further include a releasable layer (not shown) for protecting sticky layer 15.
For a high lithography resolution, the wavelength of the exposure light needs to be shortened. Currently, far ultraviolet light (KrF excimer laser (wavelength: 248nm)) has been used as exposure light, with vacuum-ultraviolet light (ArF excimer laser (193nm)) being increasingly used. Because short-wavelength exposure light carries high photon energy, pellicles become prone to photodeterioration or photodecomposition.
As pellicle membranes are made of organic material, they are particularly prone to photodeterioration or photodecomposition owing to the increasing usage of shorter wavelengths for exposure light. Photodeterioration or photodecomposition of the pellicle membrane results in a problem of reduced thickness and therefore diminished exposure light transmittance. This also triggers radical-induced cleavage and recombination of polymer chains in the pellicle membrane, resulting in changes in the refraction index of the polymer. These changes in light transmittance or refraction index leads to reduced patterning precision.
Approaches have been proposed to limit such photodeterioration or photodecomposition of pellicle membranes. One proposed approach involves treating a pellicle membrane, which is composed of an amorphous perfluoropolymer, with fluorine gas on its surface to form thereon a fluorinated layer that limits photodeterioration or photodecomposition (see Patent Literature 1). Another proposed approach involves fluorinating terminal groups of an amorphous perfluoropolymer of an amorphous perfluoropolymer-based pellicle membrane to limit photodeterioration or photodecomposition. Fluorination of the polymer's terminal groups is reportedly accomplished by blowing a mixture gas of fluorine gas and inert gas into a fluorine-containing solvent solution containing amorphous perfluoropolymer (see Patent Literature 2).